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Discussion Starter · #1 ·
Trying out two new bows. One is a 23" riser with short limbs and the other is 19" riser with medium limbs. So there is two inch difference. I have not done a draw force curve on either. Tiller is slightly off on both. But I am not sure which lift point is best? Just looking for a second opinion.
Dan
 

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Dan,

Have you measured the lift points on the two bows? You mentioned that the tillers are off. Can you see that in differences in lift point when comparing your top and bottom limbs?

It is really hard to see in the pictures, especially since you are using conventional recurves. The differences can be slight.

You have to measure the bow while drawn, rather than on a draw board to see the asymmetry since it depends on grip and finger pressure.

When I do bow balance studies I have my son measure angles and lift points at the top (from a ladder) and the bottom (sitting on the ground).

I also change grip and finger pressures to get a range of balance.

If you run draw force curves, I can process the data and create comparison charts for you. I think without draw force curves we will all be speculating.

...and you know I have been stung by incorrect speculation before--on my part. If I recall, I think it was a lift point discussion I had with you in Arizona a few years ago.
 
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Discussion Starter · #3 ·
Steve, yes I can see and feel the unbalanced limbs. At this time I am only considering the difference in bows lengths vs my draw lengths. Both bows have lifted off the grooves and seem to have the string straight out of notch. I have read that +/- 5% are not going to impact efficiency. However, one bow feel smooth and the other snappy.
Dan
 

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Dan,

I recall now that the discussion we had several years ago was about lift point and efficiency. I was pointing out how efficiency and lift point were related. This was prior to me actually doing the experiment on my shooting machine. What I found was that efficiency is actually higher for a under drawn bow, but only slightly. I tested down to 20 inches of draw. So my original hypothesis was incorrect. I think you pointed this out when I posted the results a few years ago. Remember, though, efficiency is not the same as performance. Efficiency for a bow is a measure of how much stored potential energy is converted to translational kinetic energy. While a bow that is drawn 20 inches is more efficient, it also stores considerably less energy and produces far less speed. These tests were done with conventional limbs. I bring this up since you know that I normally shoot super recurves and that was an important part of my original supposition. Here are the links to my original posts on the series of tests. I am linking from ArcheryTalk since all the charts and pictures were lost on Tradtalk. There are efficiency tests for the same arrow at different draw lengths, constant GPP, and constant stored energy/pound. So the best I can conclude is that what I reasoned to be true, was not true, and therefore, the chance that I may come up with better reasoning is suspect, without a confirming test.

(66) Grains/Pound: what does it really mean and do? Bow test results. | Archery Talk Forum
(66) Affects of draw length and arrow weight on speed and efficiency: Border XP10 | Archery Talk Forum sic (should be Effects)
(66) Expanded test procedure using a shooting machine: Border XP10 Evolution | Archery Talk Forum
 

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Discussion Starter · #5 ·
Wow, big thanks on your finding and remembering that discussion . i have a draw board, arrow weight and bow scales coming. I think i may as well replace my chronograph.
I will post back.
Dan
 

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Interesting pictures. The 23 and shorts is showing a smaller lift point angle but a harder bent in the limb, while the 19 with mediums is showing a bigger lift point angle but the limb bent is not that hard. The shorts are working harder on the 23" -> I think the performance is there, but the efficiency might be on the other combo. I wouldn't send the chrono to retirement, I would test both combos shooting a heavy arrow - at least 10gpp.
 

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Dan,

I would like to add your data to my database. I can run your numbers and send you a pdf with comparisons of the two bows. Let me know if you are interested and I will let you know what data I need and how I need it collected.
 
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Interesting pictures. The 23 and shorts is showing a smaller lift point angle but a harder bent in the limb, while the 19 with mediums is showing a bigger lift point angle but the limb bent is not that hard. The shorts are working harder on the 23" -> I think the performance is there, but the efficiency might be on the other combo. I wouldn't send the chrono to retirement, I would test both combos shooting a heavy arrow - at least 10gpp.
Draven, I don't think you were on the forum when I did the shooting machine tests in the links above. You may find some of the data interesting. I just realized that the pictures are still visible in the Tradtalk versions since they were from Photobucket. For awhile Photobucket had completely removed linked picture when they eliminated their free service. The ArcheryTalk links are higher up. The discussion is often different for this type of stuff across the two forums. As I mentioned in my post to Dan, I had a preconceived notion regarding lift point which did not prove out under testing.

(22) Grains/Inch: What does it really mean and do? Bow test results. | Trad Talk Forums sic (should say Grains/pound)
(22) Affects of draw length and arrow weight on speed and efficiency: Border XP10 | Trad Talk Forums sic (should be Effects)
(22) Expanded test procedure using a shooting machine: Border XP10 Evolution | Trad Talk Forums
 

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Archery Interchange now requires that you be a member to post. I am Amateur Barbarian over there (after English geographer and explorer Sir Richard Burton). Those posts were the first tests I ever did and when I was just starting to learn. I actually got into testing to try to explain how a super recurve worked. That is where the first derivative curve came from. That is also where my original theories on lift point originated, which ended up to be incorrect. I need to do a lot more experimentation now in order to understand it.
 

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Discussion Starter · #11 ·
I should add that one limb is wood fiberglass conventional (shorts) and the other is 25% carbon and the remaining polymeric and fiber glass (mediums). I will do it this way, then possibly switch. This will take some time because i work and all. lol
 

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Discussion Starter · #13 ·
More updates. Border 23" with Uukha longs scurve vs tribute 19" with Uukha Ex-1 mediums. My draw length is 27.75" which is 1" from full curve.
Drawforce curves for both.
Not sure you can see Scruve is about 1" higher between the two. Other than that they appear very close.
 

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Discussion Starter · #14 ·
Uukha Gobi long weight 3147 grain and tips 1281.4
Uukha Ex-1 mediums weight 2368.4 grain and tip 1158.2 grains. Using
TA Arrow speed ap on my laptop.
So, next I will be shooting. Loging Acuire Arrow Speed, Arrow speed data, power spectrum, Trajectory.
Here is my stup.
 

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I like your design. You could turn that into a shooting machine.

I am not sure about your scale. Those scales used to have auto shutoff. If you go digital you want a scale that will go up and down freely and not power save shutdown.

Many archery scales measure peak weight so you can only go one direction and cannot fine tune. I like to fine tune, but I am using a worm gear rather than a ratchet.

I like the AWS PK-110. It will do free movement, peak and hold weights and does not have a power save feature. And it will measure to 0.1 pounds, which is good for analytics.

Send me the data I will process it using the first derivate and put both on the same chart. The first derivative will highlight the differences between the two curves.
 

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Discussion Starter · #16 ·
Hank, thanks. the scale has auto shut down and peek weight. but i keep it moving so it doesn't time out and stay within a ten of a pound. I like this open design too, makes bow mounting easy. No side to side movement, only tilt for shooting distance. I will send you the data. i am looking forward in getting bow speed as well as arrow speed at target. this is something very hard to do with a chronograph with out damaging it. using sound recording let you look at tuning also.
Dan
 

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The lift point is how far you have drawn the bow when the string loses contact with the limbs. In other words, when the string is no longer in contact with the recurve. Mechanically, the limb, and the resulting leverage, increases as you draw the bow and the string is peeled off the limb. Once the string loses contact, the lever stays the same length, but you start pulling the limb tips together. This is where you start seeing stacking. The change in leverage and pull angle of the string creates the characteristic mechanical properties of a recurve bow. You essentially have a bow where the leverage increases as you draw, usually out to about 19 or 20 inches. With a super recurve it can go out to about 28 inches. If you had a bow with a siyah then you would have two lever lengths. One as you start. The other, longer lever, when the string loses contact with the siyah.
 

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Discussion Starter · #20 ·
After lift the curve becomes more linear and not as- Efficient for said recurve? So which is better 60" Dryad", 64" Tribute or 68" Borders at my 27.75" draw. I hope to answer that as well as best tuning.
 
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